Retroviral insertional mutagenesis preferentially identifies oncogenes rather than tumor suppressor (TS) genes, presumably because a single retroviral‐induced mutation is sufficient to activate an oncogene and initiate a tumor, whereas two mutations are needed to inactivate a TS gene. Here we show that TS genes can be identified by insertional mutagenesis when the screens are performed in Blm‐deficient backgrounds. Blm‐deficient mice, like Bloom syndrome patients, have increased frequencies of mitotic recombination owing to a mutation in the RecQ protein‐like‐3 helicase gene. This increased mitotic recombination increases the likelihood that an insertional mutation in one allele of a TS gene will become homozygoused by non‐sister chromatid exchange and the homozygosity of the insertion provides a marker for identifying the TS gene. We also show that known as well as novel TS genes can be identified by insertional mutagenesis in Blm‐deficient mice and identify two JmjC family proteins that contribute to genome stability in species as evolutionarily diverse as mammals and Caenorhabditis elegans.